A noise reduction system is often used to reduce noise energy in a duct or duct-like device due to some noise source. Such a noise source may be due to, but not limited to, the turbofan of an aircraft engine. Over the last several decades, much work has been done to attenuate noise generated by aircraft engines.
There are currently two available alternative technologies for reducing inlet noise in jet engines. One technology simply employs “liners” on the engine compartment which are internal coatings that absorb acoustic energy at the engine inlet. This technology is very limited in that it does not reduce noise over a large frequency range, but is mainly limited to broadband noise. Also, liners become ineffective with time because of changes in material properties due to accumulation of dirt, dust and liquids in the absorptive material. Also, the sound reduction obtained from liners is limited since the amount of reduction is directly proportional to the amount of surface treatment. Thus, if an operator wants to greatly reduce the noise using the liner, the operator must use more liner material over a larger surface area. This adds unwanted weight to the aircraft, which affects the fuel consumption of the aircraft.
Additionally, there are active noise cancellation systems known as compression type acoustic drivers, which are effective at specific frequencies. Unfortunately these devices are heavy and expensive and are not durable; i.e., the poor reliability of the moving parts would have a negative impact when used in commercial engines. Furthermore, the electrical power requirement to drive these compression drivers is much too great.
Fan noise has also been identified as a major technical concern in the development of the future engines. Future engines such as an ultra-high bypass (UHB) engine has great fuel efficiency, but at the cost of a high noise level. The introduction of ultra high bypass ratio engines having shorter inlet ducts relative to the size of the fan lessens the effectiveness of passive acoustic liners because as the frequencies decrease the acoustic wavelength increases.
Previous experiments using circumferential arrays of tubes oriented parallel to the inlet duct axis were successful. Power attenuation of up to 8 dB was achieved with the added benefit of 3 dB broadband power reduction of up to 3200 Hz. Theoretical analyses further showed the capabilities of the use of tubes angled to coincide with the propagation angle of the disturbing wave. One study showed, for a wave at 2150 Hz with a propagation angle of 40° from the duct axis, a fixed circumferential array of rigid Herschel-Quincke tubes arranged to coincide with the propagation resulted in 4.1 dB of power reduction compared to just 2.7 dB using an array of tubes oriented parallel to the duct axis.
Previous embodiments of Herschel-Quincke tube treatments have been used successfully to combat turbofan noise due to an engine running at constant speed. However, these treatments could not account for variations in frequency content and variations of the angle of propagation of the disturbance. U.S. Pat. No. 6,112,514 to Burdisso et al., the contents of which are incorporated in their entirety describes a system with Herschel-Quincke tubes to reduce frequencies at a steady state operation. U.S. patent application Ser. No. 10/343,567 filed Oct. 2, 2001 to Byrne et al, the contents of which are incorporated in its entirety, is an improvement over the Burdisso '514 patent in allowing for more adjustment to dynamic operating conditions. For years it was desired to still further noise from such apparatus as jet engines during transition periods of takeoff or landing when closest to population centers than the Burdisso and Byrne teachings provide. The instant invention addresses the problems of the prior art during acoustic noise pollution generated during transition phases that were not addressed by prior attempts to reduce noise.